Abstracts

Synthetic Aperture Magnetometry (SAM) is a spatial filtering technique for MEG data, using a minimum-variance beamformer. From the MEG signal, SAM estimates the time-course of source activity (dipole moment) from any specified location in the brain. This process is referred to as a virtual sensor (SAM-VS), behaving as if an intracranial electrode was recording cortical activity at the same site. The purpose of this study is to compare virtual epileptiform discharges observed by SAM-VS with actual discharges measured at the same sites by intracranial EEG. Data for 10 children who underwent long-term intracranial EEG and surgical excision for neocortical epilepsy were selected for this study. Their preoperative MEG was analyzed retrospectively, using SAM software (CTF Systems Inc., Port Coquitlam, Canada). Virtual sensors were computed for locations corresponding to those of the intracranial subdural grid. The topographic distribution and morphological features of epileptiform discharges seen using SAM-VS and intracranial EEG were reviewed for each patient. Main morphologic features of epileptic discharges on ECoG in 10 patients consist of focal discharges in 4, focal rhythmic (sharp) train in 2, multiple independent spikes in 2, widely distributed spike discharges in 1, and focal discharges and low-amplitude fast activities in 1. Then SAM-VS demonstrated the similar results to ECoG classifications, focal discharges in 4 out of 4, focal rhythmic (sharp) train in 2 out of 2, multiple independent spikes in 2 out of 2, and widely distributed spike discharges in 1 out of 1. In 1 patient, while low-amplitude fast activities and focal discharges were defined as the main feature on ECoG, only focal discharges were observed on SAM-VS. In 5 patients, additional minor findings which were focal discharges in centro-parietal and basal frontal regions, were identified on only ECoG. The spatial distribution and waveform morphology of virtual epileptiform discharges on MEG with SAM-VS analysis are comparable to actual discharges observed directly by intracranial EEG. We need further analysis of the interictal zone on MEG using SAM-VS to localize the epileptogenic zone, yet SAM-VS analysis can simulate the interictal epileptic behaviors prior to the epilepsy surgery.